By Robert Harveson
Sugarbeets in Nebraska and other areas of the Great Plains may be affected by a number of diseases from all pathogen groups, including fungi, bacteria, viruses and nematodes. Fungal diseases are commonly encountered causing leaf spots (Cercospora, Phoma, and Alternaria), root rots (Fusarium, Rhizoctonia, Pythium, Aphanomyces,). Bacteria are generally less problematic in this region but can cause both foliar blights, leaf spots (bacterial leaf spot) and root rots (bacterial vascular necrosis and rot – formally Erwinia root rot). Both viruses (rhizomania and beet soilborne mosaic) and nematodes (sugar cyst and false root-knot) commonly are found residing in soils, resulting in root disease problems.
The appearance and severity of specific diseases is highly dependent upon local environmental conditions, and the most destructive diseases one year may not be the same the next. Each disease is strongly influenced by environmental conditions and other factors, such as choice of variety, proximity to previously infested fields, and cultural practices that promote favorable conditions for either the pathogen or the host.
Below are eight general suggestions or reminders that should help you manage some of the common diseases in sugarbeets — and in most of the other crops on your farm. We hope that this article will also provide some assistance for growers and consultants by introducing the possible disease problems that may be encountered, how to recognize them, and to begin understanding those conditions that influence disease development for those most likely to occur. Learning to identify and distinguish between these various diseases is important for making the correct diagnosis, which then becomes a critical first step for choosing the most effective option for disease management.
Figure 1: Sample Foliar Diseases of Sugarbeet / Credit: R. Harveson
1. Integrated Pest Management (IPM) — This term is used frequently by pathologists, but for good reason. The simple idea behind this concept is that it is desirable to use multiple strategies to manage disease, not just one. IPM reduces your chances of a management failure occurring. If the genetic resistance bred into varieties doesn’t work for some unknown reason, you might still be covered because you rotated properly, are prepared to apply a fungicide, etc.
2. Know Your Enemy — There is an increasing amount of information available to help in disease identification and management. The University of Nebraska’s Panhandle Research & Extension Center attempts to disseminate information that seeks to provide accurate symptoms and key points about disease that you are likely to see. We realize that differentiating diseases can be difficult, and as a result, questions and/or submissions to extension educators, or the center’s plant pathology diagnostic lab, are encouraged.
Similarly, we will try to get critical information out as diseases are occurring. Whether it is the UNL website, radio spots or newspaper or trade journal articles such as this, it is important to stay informed.
3. Find the Enemy -- Scouting is one of the most important parts of disease management. If you are uncertain what disease(s) you have, it is very difficult to make informed decisions about specific management tools. For example, if you identify a severe epidemic of Rhizoctonia or Aphanomyces root rot, then the next time you go back in that field you might want to plant a resistant cultivar. Similarly, foliar fungicides are most effective in the early stages of an epidemic. If you let Cercospora leafspot get ahead of you, for instance, you can lose yield and profitability quickly. This is a disease that you cannot play catch-up with after it gets out of control. Furthermore, foliar fungicide applications would not be effective in managing bacterial leafspot, so correctly diagnosing the problem is critical.
4. Rotation — Sugarbeets generally perform better after rotations. Three- to four-year rotations are typically recommended, depending on crops grown within the given rotation. Most of the soilborne root pathogen problems will be diminished somewhat by this length of rotation. However, they don’t disappear completely, as these pathogens have built-in survival mechanisms to persist for long periods in soil, even under adverse conditions.
Figure 2: Sample Root Rot Diseases of Sugarbeet. Credit: R. Harveson
5. Varietal Resistance -- For a number of reasons, the selection of a particular cultivar is perhaps the most important decision you make. If you have a field history of rhizomania, planting a cultivar with some resistance will likely be the most cost-effective disease management tool at your disposal. Just remember that “resistance” does not guarantee complete control, as none of the resistant varieties offer “immunity.” In the case of rhizomania, the resistance does not prohibit infection and replication of the viral pathogen. The plant simply grows and yields well in spite of being potentially infected. We are currently screening soil samples for the Western Sugar Cooperative, looking for the incidence or presence of pathogen isolates that may be able to overcome the genetic resistance incorporated into all the varieties used in this region.
6. Seed Treatments -- Sugarbeets may be treated with one or more of several fungicides for protection against certain fungal pathogens such as Rhizoctonia, Aphanomyces or Pythium. These products will not last for the entire season, but may be useful for reducing damping-off and stand reduction problems during the most important part of the plant’s development. Establishment of a healthy stand is critical for a healthy crop. However, integrating a seed treatment with a resistant cultivar or a later fungicide application is a good example of employing the concept of IPM.
7. Foliar Fungicides -- On sugarbeet crops in Nebraska and other areas of the Central Great Plains, the most important disease to manage with fungicides is Cercospora leafspot (CLS). Although other common leaf diseases (Alternaria, Phoma and bacterial leafspots) (Figure 1) are readily found in beet fields in this region, only CLS is damaging enough to economically justify making fungicide applications. This makes a correct diagnosis particularly important. For example, if growers mistakenly thought they had an epidemic of CLS and it turned out to be bacterial leafspot, they would waste their money applying a product with no chance of being effective. Conversely, if CLS was confused with Phoma leafspot and was not sprayed with a fungicide (knowing that Phoma rarely requires treatment), the grower would very likely incur a loss due to the failure to treat CLS.
8. Stay Engaged and Adapt — The changes in the world of agriculture occur quickly. In the future, we would also anticipate that diseases and disease management will be different. We may have new diseases (or races) to worry about, new resistance in hybrids to combat them, and new fungicides or other products to use — and new recommendations to go with them. The more knowledge you have about disease management, the more likely you are to be able to manage diseases in a changing world.
Robert Harveson is extension plant pathologist with the University of Nebraska’s Panhandle Research & Extension Center, Scottsbluff.
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